1 Answer
1

I found a paper from 2005 analyzing the current sate of potentially polluting wrecks from World War 2. It has the information we need:

The database includes non-tank vessels of at least 400 gross tonnage (GT) holding petroleum-based oil as fuel/bunkers (and for operations) and tank vessels of at least 150 GT holding petroleum-based oil as cargo and fuel/bunkers (and for operations).... The resulting database includes 8,569 ... wrecks [containing oil], with 1,583 tank vessels and 6,986 non-tank vessels. Estimates of the likely volume of oil remaining onboard these wrecks were made, particularly when the volume of oil onboard was not known. A high estimate was calculated assuming that a tank vessel had at least 80 percent of its cargo capacity onboard, and bunkers were assumed to be 70 percent full.... The [result was] a high estimate of 20.4 million tonnes (6 billion gallons).

A low estimate would be 1.5 billion gallons (assuming 20 percent full at time of wreck). A reasonable average would be 3.8 billion gallons of oil spilled in maritime waters during World War 2. This estimate includes both military vessels and non-military vessels sunk during World War 2. It does not include oil spilled in rivers or freshwater lakes.